Microwave synthesis of high-quality and uniform 4 nm ZnFe₂O₄ nanocrystals for application in energy storage and nanomagnetics

• Christian Suchomski,
• Ben Breitung,
• Ralf Witte,
• Michael Knapp,
• Sondes Bauer,
• Tilo Baumbach,
• Christian Reitz and
• Torsten Brezesinski

Beilstein J. Nanotechnol. 2016, 7, 1350–1360, doi:10.3762/bjnano.7.126

• kept at about 3.0 V with respect to Li+/Li and the other was lithiated until a potential of 0.85 V was reached, which is within the main plateau. FeO (wüstite), Fe3O4 (magnetite) and α-Fe2O3 (hematite) were used as the reference materials for Fe(II) in cubic and cubic/spinel and Fe(III) in trigonal

Improved biocompatibility and efficient labeling of neural stem cells with poly(L-lysine)-coated maghemite nanoparticles

• Igor M. Pongrac,
• Marina Dobrivojević,
• Lada Brkić Ahmed,
• Michal Babič,
• Miroslav Šlouf,
• Daniel Horák and
• Srećko Gajović

Beilstein J. Nanotechnol. 2016, 7, 926–936, doi:10.3762/bjnano.7.84

• FeCl2 was added and the mixture poured into 36 mL of 0.5 M NH4OH. The formed magnetite coagulate was left to grow for 15 min, after which it was magnetically separated, repeatedly washed with ultrapure water and passed through a 0.22 µm PTFE Millex membrane filter (Millipore) to remove all impurities

• remaining after the synthesis. Under sonication 1.5 mL of 0.1 M sodium citrate was added, after which magnetite was oxidized by addition of 1 mL of 5% sodium hypochlorite solution. To coat nanoparticles 0.2 mL of aqueous poly(L-lysine) solution (1 mg/mL) was added dropwise with stirring to 10 mL of primary

Hemolysin coregulated protein 1 as a molecular gluing unit for the assembly of nanoparticle hybrid structures

• Tuan Anh Pham,
• Andreas Schreiber,
• Elena V. Sturm (née Rosseeva),
• Stefan Schiller and
• Helmut Cölfen

Beilstein J. Nanotechnol. 2016, 7, 351–363, doi:10.3762/bjnano.7.32

• gluing unit for the assembly of often linear, hybrid structures of plasmonic gold (Au NP), magnetite (Fe3O4 NP), and cobalt ferrite nanoparticles (CoFe2O4 NP). Furthermore, the assembly of Au NPs into linear structures using Hcp1_cys3 is investigated by UV–vis spectroscopy, TEM and cryo-TEM. One key

• Hcp1_cys3 as a connecting unit is extended to magnetite NPs (Fe3O4 NPs) and cobalt ferrite NPs (CoFe2O4 NPs). The synthesis of magnetite NPs with a size of about 8 nm and the ligand exchange followed the protocol of Cabrera et al. [36], with results as shown in the TEM images of Figure S4, Supporting

• TEM images (Figure S5, Supporting Information File 1). These results indicate that the prealignment of Fe3O4 NPs in an external magnetic field is essential for the linear arrangement of the NPs chain formation. On the other hand, TEM investigations of the hybrid material show a network of magnetite

Surface coating affects behavior of metallic nanoparticles in a biological environment

• Darija Domazet Jurašin,
• Marija Ćurlin,
• Ivona Capjak,
• Tea Crnković,
• Marija Lovrić,
• Michal Babič,
• Daniel Horák,
• Ivana Vinković Vrček and
• Srećko Gajović

Beilstein J. Nanotechnol. 2016, 7, 246–262, doi:10.3762/bjnano.7.23

• using ultracentrifugation. Three different maghemite nanoparticles (γ-Fe2O3NPs), uncoated, coated with poly(L-lysine) and D-mannose, were prepared by coprecipitation of FeCl2 and FeCl3 using ammonium hydroxide, followed by the oxidation of the resulting magnetite with sodium hypochlorite [46][47]. The

Simultaneous cancer control and diagnosis with magnetic nanohybrid materials

• Reza Saadat and
• Franz Renz

Beilstein J. Nanotechnol. 2016, 7, 121–125, doi:10.3762/bjnano.7.14

• Reza Saadat Franz Renz Institute for Inorganic Chemistry, Leibniz University Hannover, Callinstr. 3–9, 30169 Hannover, Germany 10.3762/bjnano.7.14 Abstract Coated magnetite nanoparticles were linked to 68Ga complexes used in the positron emission tomography (PET) for a new technical approach to

• of the particles in the tissue. With this novel method of combining detection and treatment simultaneously, the amount of medical exposure could be minimized for the patient. The results demonstrate that magnetite nanoparticles can effectively be functionalized with PET isotopes and pH sensitive

• complexes in order to use them as a new type of radiopharmaceuticals. Keywords: biocompatible nanoparticles; cancer control; cancer diagnosis; magnetite nanoparticles; positron emission tomography; Introduction In the field of diagnostic investigation of metabolic diseases imaging methods based on

An adapted Coffey model for studying susceptibility losses in interacting magnetic nanoparticles

• Mihaela Osaci and
• Matteo Cacciola

Beilstein J. Nanotechnol. 2015, 6, 2173–2182, doi:10.3762/bjnano.6.223

• basis for practical magnetic hyperthermia than small nanoparticles [6]. Usually, the magnetite nanoparticles used in hyperthermia have a diameter above 53 nm but below the diameter above which objects are no more considered to be “nano” (100 or 200 nm, according to the bibliographic sources). Actually

• , magnetite is considered the most favourable material in magnetic hyperthermia. At about 30 nm particle diameter the behaviour of magnetite nanoparticles changes from single-domain to multi-domain state [7], representing the critical dimension of magnetite-based nanoparticles. The tendency for agglomeration

• Numerical simulations We considered a system with spherical nanoparticles made of uncoated magnetite, with the following characteristics: density ρ = 5180 kg/m3 [3]; saturation magnetization Ms = 4.46·105 A/m [3]; uniaxial magnetic anisotropy with anisotropy constant Keff = 25·103 J/m3 [3]; random

A facile method for the preparation of bifunctional Mn:ZnS/ZnS/Fe₃O₄ magnetic and fluorescent nanocrystals

• Houcine Labiadh,
• Tahar Ben Chaabane,
• Romain Sibille,
• Lavinia Balan and
• Raphaël Schneider

Beilstein J. Nanotechnol. 2015, 6, 1743–1751, doi:10.3762/bjnano.6.178

• → 6A1 Mn2+ transition. The PL quantum yield (QY) and the remnant magnetization can be regulated by varying the thickness of the magnetic shell. The results showed that an increase in the thickness of the Fe3O4 magnetite layer around the Mn:ZnS/ZnS core reduced the PL QY but improved the magnetic

• properties of the composites. Nevertheless, a good compromise was achieved in order to maintain the dual modality of the nanocrystals, which may be promising candidates for various biological applications. Keywords: aqueous synthesis; bifunctional nanocrystals; magnetite; Mn-doped ZnS; quantum dots

• correspond to the magnetite Fe3O4 spinel-like structure (Figure 2f) [26][27]. Note that this sample was prepared using FeCl3 as a precursor in the presence of the MPA ligand, which probably also acts as a reducing agent to provide the necessary amount of Fe2+ to form magnetite. For the Mn:ZnS/ZnS and Mn:ZnS

Synthesis, characterization and in vitro biocompatibility study of Au/TMC/Fe₃O₄ nanocomposites as a promising, nontoxic system for biomedical applications

• Hanieh Shirazi,
• Maryam Daneshpour,
• Soheila Kashanian and
• Kobra Omidfar

Beilstein J. Nanotechnol. 2015, 6, 1677–1689, doi:10.3762/bjnano.6.170

• engineering, as well as the in the design of sensors and biosensors [11][12][13][14][15][16][17]. Although all nanoparticles containing a magnetic core are considered as magnetic nanoparticles, the most commonly used are iron oxide nanoparticles, which are mostly synthesized in the form of magnetite (Fe3O4

Structural and magnetic properties of iron nanowires and iron nanoparticles fabricated through a reduction reaction

• Marcin Krajewski,
• Wei Syuan Lin,
• Hong Ming Lin,
• Katarzyna Brzozka,
• Sabina Lewinska,
• Natalia Nedelko,
• Anna Slawska-Waniewska,
• Jolanta Borysiuk and
• Dariusz Wasik

Beilstein J. Nanotechnol. 2015, 6, 1652–1660, doi:10.3762/bjnano.6.167

• ] and 130 emu/g [24], respectively, and knowing that amorphous iron oxides exhibit paramagnetic behaviour [20][22], and considering the magnetization of distorted iron oxides to be about the value of bulk magnetite (Fe3O4) of around 92 emu/g [25], the estimated magnetizations (Mcal.) of the investigated

• magnetite [12]. This is also consistent with another report, in which the authors have demonstrated that with increasing core size of iron nanostructures the oxide shell is composed almost entirely of magnetite [26]. Applying Equation 2, the estimated magnetization of iron nanowires and iron nanoparticles

Thermal treatment of magnetite nanoparticles

• Beata Kalska-Szostko,
• Urszula Wykowska,
• Dariusz Satula and
• Per Nordblad

Beilstein J. Nanotechnol. 2015, 6, 1385–1396, doi:10.3762/bjnano.6.143

• /bjnano.6.143 Abstract This paper presents the results of a thermal treatment process for magnetite nanoparticles in the temperature range of 50–500 °C. The tested magnetite nanoparticles were synthesized using three different methods that resulted in nanoparticles with different surface characteristics

• stability properties were compared. In this study, two sets of unmodified magnetite nanoparticles were used where crystallinity was as determinant of the series. For the third type of particles, a Ag shell was added. By comparing the coated and uncoated particles, the influence of the metallic layer on the

• the fabrication methods determine, to some extent, the sensitivity of the nanoparticles to external factors. Keywords: high temperature corrosion; internal oxidation; IR spectroscopy; metal matrix composites; Mössbauer spectroscopy; X-ray diffraction; Introduction Nanostructured magnetite has become

Silica micro/nanospheres for theranostics: from bimodal MRI and fluorescent imaging probes to cancer therapy

• Shanka Walia and
• Amitabha Acharya

Beilstein J. Nanotechnol. 2015, 6, 546–558, doi:10.3762/bjnano.6.57

• composed of La0.75Sr0.25MnO3 (LSMO) perovskite magnetite NPs encapsulated in a silica layer and to these NPs, a fluorescent FITC–APS (F) complex was covalently attached. These LSMO@SiF NPs were again coated with silica either by direct treatment with TEOS to form LSMO@SiF@Si-u or by reacting the

• magnetic NPs doped inside silica was found to be 7.0 emu/g, which was far less than the saturation magnetization of the magnetite/maghemite NPs used for the preparation of these core–shell spheres (54 emu/g). This decrease in magnetic saturation was attributed to the presence of a thick silica shell in the

• nanocomposites suggested the presence of the aggregates inside the cells. Kim et al. [48] reported the synthesis of magnetic and fluorescent NPs, encapsulated inside silica through a sol–gel process. The synthesis involved the encapsulation of oleic acid-capped magnetite NPs and CdSe/ZnS QDs, inside the silica

Hematopoietic and mesenchymal stem cells: polymeric nanoparticle uptake and lineage differentiation

• Ivonne Brüstle,
• Thomas Simmet,
• Gerd Ulrich Nienhaus,
• Katharina Landfester and
• Volker Mailänder

Beilstein J. Nanotechnol. 2015, 6, 383–395, doi:10.3762/bjnano.6.38

• capacity of hHSCs and hMSCs to obtain a deeper knowledge of the interaction of stem cells and nanoparticles. As model systems of nanoparticles, two sets of either bioinert (polystyrene without carboxylic groups on the surface) or biodegradable (PLLA without magnetite) particles were analyzed. Flow

• were chosen for this study: non-functionalized polystyrene (PS) and carboxy-functionalized polystyrene (PS–COOH). PS–COOH particles are biocompatible, but nondegradable particles whereas poly(L-lactide) particles without (PLLA) and with magnetite (PLLA–Fe) are biocompatible and biodegradable. All

• particles were fluorescently labeled with the fluorescent dye N-(2,6-diisopropylphenyl)perylene-3,4-dicarboximide (PMI). The magnetite was encapsulated for magnetic resonance purposes. The polymeric nanoparticles used in this work were obtained by miniemulsion polymerization (non-functionalized and

Multifunctional layered magnetic composites

• Maria Siglreitmeier,
• Baohu Wu,
• Tina Kollmann,
• Martin Neubauer,
• Gergely Nagy,
• Dietmar Schwahn,
• Vitaliy Pipich,
• Damien Faivre,
• Dirk Zahn,
• Andreas Fery and
• Helmut Cölfen

Beilstein J. Nanotechnol. 2015, 6, 134–148, doi:10.3762/bjnano.6.13

• Potsdam, Germany 10.3762/bjnano.6.13 Abstract A fabrication method of a multifunctional hybrid material is achieved by using the insoluble organic nacre matrix of the Haliotis laevigata shell infiltrated with gelatin as a confined reaction environment. Inside this organic scaffold magnetite nanoparticles

• the particle size. Simulation studies show the potential of collagen and chitin to act as nucleators, where there is a slight preference of chitin over collagen as a nucleator for magnetite. Colloidal-probe AFM measurements demonstrate that introduction of a ferrogel into the chitin matrix leads to a

• certain increase in the stiffness of the composite material. Keywords: bio-inspired mineralization; biomineralization; chitin; ferrogel; hybrid materials; magnetite; nacre; Introduction Biominerals, which are organic–inorganic hybrids and highly sophisticated materials with optimal assimilated

Nanoencapsulation of ultra-small superparamagnetic particles of iron oxide into human serum albumin nanoparticles

• Matthias G. Wacker,
• Mahmut Altinok,
• Stephan Urfels and
• Johann Bauer

Beilstein J. Nanotechnol. 2014, 5, 2259–2266, doi:10.3762/bjnano.5.235

• molecules [14]. Since the HSA molecule is negatively charged during desolvation process, positively charged compounds demonstrate a high affinity to the matrix material [13]. Therefore, magnetite nanoparticles have been modified in order to increase charge–charge interactions between USPIO and the matrix

• , particle diameter, size distribution, particle shape, and surface charge have been investigated. Characterization of magnetite core particles The magnetite structure was confirmed by the recorded powder X-ray diffraction (PXRD) patterns (Figure 1) exactly matching with the standard (ICDD card no. 19-629

• = 6 nm). Additionally, the specific (mass dependant) magnetization of the particles was determined at a temperature of 300 K (Figure 2). Surface modification of magnetite nanoparticles Iron oxide nanoparticles were chemically modified by using a combination of citrate and tetramethylammonium hydroxide

Imaging the intracellular degradation of biodegradable polymer nanoparticles

• Anne-Kathrin Barthel,
• Martin Dass,
• Melanie Dröge,
• Jens-Michael Cramer,
• Daniela Baumann,
• Markus Urban,
• Katharina Landfester,
• Volker Mailänder and
• Ingo Lieberwirth

Beilstein J. Nanotechnol. 2014, 5, 1905–1917, doi:10.3762/bjnano.5.201

• biological environments, intracellular degradation processes have been examined only to a very limited extent. PLLA nanoparticles with an average diameter of approximately 120 nm were decorated with magnetite nanocrystals and introduced into mesenchymal stem cells (MSCs). The release of the magnetite

• particles from the surface of the PLLA nanoparticles during the intracellular residence was monitored by transmission electron microscopy (TEM) over a period of 14 days. It was demonstrated by the release of the magnetite nanocrystals from the PLLA surface that the PLLA nanoparticles do in fact undergo

• ., number of detached magnetite crystals, and the number of nanoparticles in one endosome), we demonstrate the importance of TEM studies for such applications in addition to fluorescence studies (flow cytometry and confocal laser scanning microscopy). Keywords: biodegradation; mesenchymal stem cells; PLLA

Cathode lens spectromicroscopy: methodology and applications

• T. O. Menteş,
• G. Zamborlini,
• A. Sala and
• A. Locatelli

Beilstein J. Nanotechnol. 2014, 5, 1873–1886, doi:10.3762/bjnano.5.198

• -sized magnetite islands was found to be about 1 nm, which corresponds to two unit cells [72]. XMCD-PEEM measurements on these ultrathin islands (seen in Figure 11a) show that magnetite preserves its ferrimagnetic properties at 1 nm thickness up to 520 K (above which the morphology changes irreversibly

• ). This observation corresponds to the thinnest magnetite crystal that shows magnetism. Beyond the self-organized crystal shapes at the micrometer scale, epitaxial iron-oxide films provide a variety of complex surface reconstructions at the atomic scale as usual for oxide surfaces [74]. Fe3O4 films on Pt

• thermal reduction with LEEM and LEED was crucial in understanding the reversible changes in thin magnetite and hematite films grown on several substrates [75]. In particular, annealing in UHV led to substrate-dependent transformations of the iron oxide thin film: from hematite to magnetite on a Pt(111

Influence of surface-modified maghemite nanoparticles on in vitro survival of human stem cells

• Michal Babič,
• Daniel Horák,
• Lyubov L. Lukash,
• Tetiana A. Ruban,
• Yurii N. Kolomiets,
• Svitlana P. Shpylova and
• Oksana A. Grypych

Beilstein J. Nanotechnol. 2014, 5, 1732–1737, doi:10.3762/bjnano.5.183

• (magnetite Fe3O4 or maghemite γ-Fe2O3) are their simple preparation and their magnetic properties, which are necessary for detection. Moreover, it is convenient that iron oxides are readily metabolized in the body. From this point of view, quantum dots are disqualified due to their toxicity. Like in every

A sonochemical approach to the direct surface functionalization of superparamagnetic iron oxide nanoparticles with (3-aminopropyl)triethoxysilane

• Bashiru Kayode Sodipo and
• Azlan Abdul Aziz

Beilstein J. Nanotechnol. 2014, 5, 1472–1476, doi:10.3762/bjnano.5.160

• silanol groups to form siloxane (Si–O–Si) bonds. The successful grafting of the APTES molecules on the SPION is verified through FTIR analysis (Figure 1). In both spectra, the peaks of the magnetite (Fe–O–Fe) band split into two. The energy absorbed at 628 and 573 cm−1 corresponds to the first band. The

• second band is observed at 441 cm−1 which is assigned to the Fe–O bond of bulk magnetite at 375 cm−1. The formation of SPION–APTES is observed in Figure 1b by the 1109 and 1044 cm−1 peaks, which can be assigned to the Si–O–Si bond. The IR absorption lines due to the adsorption of water molecules to the

• standard of either magnetite or maghemite (cubic phase) XRD spectrum. However, the assignment to one of these phases on the basis of the XRD is difficult owing to their related structures. The absence of peaks at 110 and 104 corresponding to goethite and hematite in the spectrum indicate that the co

PEGylated versus non-PEGylated magnetic nanoparticles as camptothecin delivery system

• Paula M. Castillo,
• Mario de la Mata,
• Maria F. Casula,
• José A. Sánchez-Alcázar and
• Ana P. Zaderenko

Beilstein J. Nanotechnol. 2014, 5, 1312–1319, doi:10.3762/bjnano.5.144

• coating further reduces SPION cytotoxicity [36]. Moreover, PEGylated CPT has demonstrated its capability to lock the CPT E ring in its desired active lactone configuration [37]. Herein, we report a simple method to synthesise PEG-coated ultrasmall magnetite (USM) nanoparticles, and we examine the ability

• (Figure 2c). Further insights into the crystalline structure were obtained from X-ray diffraction (XRD) patterns. Figure 3 reports the XRD pattern of our USM nanoparticles compared to a reference of a commercial magnetite standard. Although it is not possible to unambiguously ascribe the obtained pattern

• for USM nanoparticles to magnetite rather than to the isostructural spinel ferric iron oxide (maghemite), the XRD sample of the USM nanoparticles is consistent with the formation of magnetite. The peak broadening of the XRD pattern of the USM sample is in agreement with its nanocrystalline form. In

Antimicrobial nanospheres thin coatings prepared by advanced pulsed laser technique

• Alina Maria Holban,
• Valentina Grumezescu,
• Alexandru Mihai Grumezescu,
• Bogdan Ştefan Vasile,
• Roxana Truşcă,
• Rodica Cristescu,
• Gabriel Socol and
• Florin Iordache

Beilstein J. Nanotechnol. 2014, 5, 872–880, doi:10.3762/bjnano.5.99

• -chitosan-magnetite-eugenol (PLA-CS-Fe3O4@EUG) nanospheres by matrix assisted pulsed laser evaporation (MAPLE). Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) investigation proved that the homogenous Fe3O4@EUG nanoparticles have an average diameter of about 7 nm, while the PLA

• ) bacteria strains. Moreover, the obtained nano-coatings showed a good biocompatibility and facilitated the normal development of human endothelial cells. These nanosystems may be used as efficient alternatives in treating and preventing bacterial infections. Keywords: antimicrobial; chitosan; magnetite

• nanoparticles; nanospheres; P. aeruginosa; polylactic acid; S. aureus; Introduction Driven by more and more microbial antibiotic resistance, alternative therapeutic approaches are emerging [1][2][3][4]. Polar and nonpolar, functionalized and non-functionalized magnetite nanostructures have proven successfully

Manipulation of isolated brain nerve terminals by an external magnetic field using D-mannose-coated γ-Fe₂O₃ nano-sized particles and assessment of their effects on glutamate transport

• Tatiana Borisova,
• Natalia Krisanova,
• Arsenii Borуsov,
• Roman Sivko,
• Ludmila Ostapchenko,
• Michal Babic and
• Daniel Horak

Beilstein J. Nanotechnol. 2014, 5, 778–788, doi:10.3762/bjnano.5.90

• of iron salts, namely FeCl2 and FeCl3, by rapid increase of pH by ammonia. Similarly as described in [12], this was followed by the oxidation of the resulting magnetite (Fe3O4) with sodium hypochlorite producing maghemite (γ-Fe2O3), which is chemically more stable than Fe3O4. This is in contrast to

• magnetite, which undergoes spontaneous oxidation by air. Proof of the maghemite by Mössbauer spectroscopy and powder X-ray diffraction, as well as its magnetic properties were described in our previous reports [15][16]. The neat γ-Fe2O3 nanoparticles were used in control experiments with cells or were used

• solution (350 mL) under a nitrogen atmosphere. The resulting coagualate of magnetite was left to grow for 45 min under nitrogen atmosphere, then magnetically separated and repeatedly (7–10×) washed (peptized) with Q-water to remove all impurities (including NH4Cl) remaining after the synthesis. Finally

Thermal stability and reduction of iron oxide nanowires at moderate temperatures

• Annalisa Paolone,
• Marco Angelucci,
• Stefania Panero,
• Maria Grazia Betti and
• Carlo Mariani

Beilstein J. Nanotechnol. 2014, 5, 323–328, doi:10.3762/bjnano.5.36

• infrared spectrum of the sample heated at 470 K (sample 3) is very similar to that of sample 2, while after the thermal treatment at 560 K (sample 4), the minimum around 700 cm−1 becomes deeper and the transmittance below 600 cm−1 decreases, which strongly resembles the infrared spectrum of magnetite

• treatment at T2 = 560 K. Moreover, at T2 a significant part of the sample is transformed into magnetite. We remark that the IR spectra are measured in transmission mode, so that they probe the whole thickness of the NW powders and are not limited to their surface. This issue is important to compare the IR

Magnetic interactions between nanoparticles

• Steen Mørup,
• Mikkel Fougt Hansen and
• Cathrine Frandsen

Beilstein J. Nanotechnol. 2010, 1, 182–190, doi:10.3762/bjnano.1.22

• direction in a double chain of 24 ~70 nm magnetite (Fe3O4) particles in magnetotactic bacteria [33], and it has resolved magnetic flux closure in small rings of 5–7 Co particles with a diameter of about 25 nm [32]. Influence of exchange coupling between nanoparticles on magnetic relaxation In a perfect

Magnetic nanoparticles for biomedical NMR-based diagnostics

• Huilin Shao,
• Tae-Jong Yoon,
• Monty Liong,
• Ralph Weissleder and
• Hakho Lee

Beilstein J. Nanotechnol. 2010, 1, 142–154, doi:10.3762/bjnano.1.17

• ][37][38][39][40][41][42]. CLIO nanoparticles contain a superparamagnetic iron oxide core (3–5 nm monocrystalline iron oxide) composed of ferrimagnetic magnetite (Fe3O4) and/or maghemite (γ-Fe2O3). The metallic core is subsequently coated with biocompatible dextran, before being cross-linked with

Uniform excitations in magnetic nanoparticles

• Steen Mørup,
• Cathrine Frandsen and
• Mikkel Fougt Hansen

Beilstein J. Nanotechnol. 2010, 1, 48–54, doi:10.3762/bjnano.1.6

• Equation 1, and the latter approximation is valid at low temperatures. The linear temperature dependence of the magnetization in nanoparticles was first observed by Mössbauer spectroscopy studies of magnetite (Fe3O4) nanoparticles [3], but it has later been studied in nanoparticles of several other

• the relaxation is fast compared to the timescale of Mössbauer spectroscopy, and the observed magnetic hyperfine field is then given by where B0 is the saturation hyperfine field. Figure 3 shows the temperature dependence of the magnetic hyperfine field of three samples of magnetite (Fe3O4

• dependence of the magnetization. TB is the superparamagnetic blocking temperature and TC is the Curie temperature. The reduced average magnetic hyperfine field as a function of temperature for particles of magnetite with sizes of 6 nm, 10 nm and 12 nm. The solid lines are the best linear fits to the